has developmental contribution from has developmental contribution from endoderm yolk sac SCTID:362845002 NCIT:C14128 EHDAA2:0002212 CALOHA:TS-1130 MESH:D015017 UBERON:0001040 GAID:1301 EFO:0003078 EMAPA:16085 BTO:0001471 VHOG:0000830 EHDAA:164 FMA:87180 A sac-like expansion of the ventral wall of the intestine, narrowed into a yolk stalk near the body[Hyman's]. Membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals. It functions as the developmental circulatory system of the human embryo, before internal circulation begins. In the mouse, the yolk sac is the first site of blood formation, generating primitive macrophages and erythrocytes[WP]. Wikipedia:Yolk_sac splanchnic layer of lateral plate mesoderm extraembryonic tissue extraembryonic membrane epithelial sac